1. Field of the Invention
The invention is related to microelectromechanical systems (MEMS) devices.
2. Description of the Related Art
In general, a MEMS transducer converts energy between electrostatic and mechanical forms. MEMS transducers may be used as both sensors that convert motion into electrical energy (accelerometers, pressure sensors, etc.) and actuators that convert electrical signals to motion (comb drive, micromirror devices, oscillators). MEMS devices using capacitive transducers are easy to manufacture and result in low noise and low power consumption sensors and/or actuators.
Capacitive sensing is based on detecting a change in capacitance of a capacitor. If a known voltage is applied across the capacitor (e.g., fixed DC bias voltages applied to the mass and electrodes of a MEMS device), changes in current due to capacitive variations will appear in response to motion of one plate of the capacitor relative to another plate of the capacitor. Similarly, capacitive actuation is based on variation in capacitance of the MEMS device. For example, a DC operating point is established by applying a DC bias voltage across the capacitor and an AC signal changes the capacitance causing changes in force on a plate of the capacitor. Transduction of a MEMS device is based on the voltage difference across the transduction gap (i.e., the voltage difference between the mass and the electrode). However, the transduction gap may vary as a function of environmental factors (e.g., temperature, strain, and aging), thereby changing the capacitance with respect to time, which may affect the spring constant (i.e., spring stiffness) associated with a MEMS device, which is typically modeled as a mass-spring damper system. In general, a change in the electrode capacitance affects the equivalent spring stiffness through electrostatic pulling, which affects the resonant frequency of the MEMS device. MEMS devices targeting applications requiring high-precision (e.g., resonators having resonant frequency specifications required to be within +/−10 parts-per-million (ppm)) may not achieve the target specification due to effects of environmental factors on the resonant frequency.
Accordingly, techniques for reducing or eliminating effects of strain on a MEMS device are desired.